1. Field of the Invention
The present invention relates to an imaging apparatus, driving method thereof, and imaging system.
2. Description of the Related Art
Recently, with the spread of HDTV (High Definition Television), there has been a demand for an imaging apparatus which takes moving images with an aspect ratio of 16:9. On the other hand, still images are expected to be taken with an aspect ratio of 4:3 because photo paper has an aspect ratio of approximately 4:3. Thus, to implement an imaging apparatus which supports both moving and still images, it is necessary to be able to switch the aspect ratio between the 16:9 mode and the 4:3 mode. Such an imaging apparatus has an imaging area with an aspect ratio of 4:3 as a whole and reads out all pixel signals in the 4:3 mode. In the 16:9 mode, the imaging apparatus reads out signals of only pixels in the center (a readout region in which signals are read out from pixels), and does not read out signals of pixels at top and bottom (non-readout regions in which signals are not read out from pixels).
Japanese Patent Laid-Open No. 2000-350103 discloses a technique for preventing photoelectric charge from overflowing from pixels in non-readout regions into pixels in a readout region by resetting photoelectric conversion elements of the pixels in the non-readout regions. According to Japanese Patent Laid-Open No. 2000-350103, all the pixels have a vertical switching MOS.
On the other hand, to reduce pixel size, it is necessary to reduce the number of MOS transistors per pixel, by eliminating the vertical switching MOS. For that, Japanese Patent Laid-Open No. 11-112018 discloses a technique for selecting pixels by changing an operating point of an input terminal (floating diffusion: FD unit) of a signal amplification unit. That is, the FD units of selected pixels are set to an on-state electric potential and the FD units of deselected pixels are set to an off-state electric potential. This makes it possible to read out outputs of only the selected pixels to column signal lines.
When performing a 16:9 mode operation using the pixel circuit described in Japanese Patent Laid-Open No. 11-112018 and the driving method described in Japanese Patent Laid-Open No. 2000-350103, a simple combination of the two techniques does not enable proper operation. When resetting the photoelectric conversion elements of the pixels in non-readout regions in the 16:9 mode, it is necessary to preset the FD units of the pixels to an electric potential needed to reset the photoelectric conversion elements. Also, since the pixels in the non-readout regions are not selected in the 16:9 mode, the FD units of the pixels must be set to the off-state electric potential.
When, for example, amplifier transistors are N-MOSFETs, if the off-state electric potential is lower than the level needed to reset the photoelectric conversion elements, electric potential setting of the FD units of the pixels in the non-readout regions causes inconsistency in the 16:9 mode. If the FD units of the pixels are set to the electric potential needed to reset the photoelectric conversion elements, the electric potential of the FD units is inevitably set higher than the off-state electric potential. This causes amplifier transistors in the non-readout regions to turn on, causing the pixel signals to be read out from the non-readout regions to the column signal lines.
Conversely, if the FD units of the pixels in the non-readout regions are set to the off-state electric potential in the 16:9 mode, the electric potential of the FD units is inevitably set lower than the level needed to reset the photoelectric conversion elements. This makes it impossible to reset the photoelectric conversion elements of the pixels in the non-readout regions. This in turn may cause photoelectric charge to leak from the pixels in the non-readout regions into the pixels in the readout region.